The proposed research tests whether natural selection contributes to the maintenance of polymorphism within species and to genetic divergence between species, and if selection is operating to distinguish between different kinds of selection. The analysis relies jointly on a powerful new strategy for determining the DNA sequences of many alleles of the same gene and on a new set of statistical tests for distinguishing evolutionary forces. By comparing nucleotide polymorphism within species to divergence between species the statistical tests allow genetic drift, positive selection (adaptive change), negative selection (adaptive constraint) and balancing selection (heterozygote advantage) to be distinguished one from another. The tests will also address whether amino acid substitutions in protein evolution are governed by genetic drift or by positive selection. The goal of the study is to understand the evolutionary cause of two major patterns of protein variation: loci with no polymorphism and loci with Intermediate frequency alleles. Nucleotide polymorphism levels will be determined for six gene loci in each of three related Drosophila species. The loci have been chosen so that specific predictions can be made about the mode of selection affecting their evolution. The kinds of genetic variation in proteins in natural populations, and the forces of selection operating on them is of direct interest to problems of genetic disease and to variation In response to conditions of the environment.